1. Field of the Invention
The present invention relates generally to a rare-earth doped optical fiber module and a manufacturing method therefor, and more particularly to an erbium-doped optical fiber (EDF) module and a manufacturing method therefor.
2. Description of the Related Art
An optical communication system using an optical amplifier for directly amplifying an optical signal has become general, and an EDF module is used as a basic component of such an optical amplifier having an optical amplifying function. FIG. 1 shows an EDF module 1 in the related art. As shown in FIG. 1, the EDF module 1 is configured by winding an Er doped optical fiber 4 around a metal reel 2. By adjusting the length of the Er doped optical fiber 4, the amplification characteristic is set to a required value. Further, normal single-mode optical fibers are spliced to the opposite ends of the Er doped optical fiber 4.
The spliced portion between the Er doped optical fiber 4 and each single-mode optical fiber is protected by dropping a UV curing resin to the spliced portion and directing UV radiation to the UV curing resin to cure the same. This process is referred to as recoating of the spliced portion. After protecting each spliced portion by this recoating process, the single-mode optical fibers are also wound around the reel for storage. In an EDF module for a wavelength division multiplexing (WDM) amplifier, uniform temperature control of an Er doped optical fiber is performed by utilizing heat conduction of a metal reel as measures against the temperature dependence of amplification characteristic.
The related art EDF module 1 using the metal reel 2 has such a problem that the loss due to stress generated in winding the Er doped optical fiber 4 is increased. Accordingly, the loss contributing to the amplification characteristic of the Er doped optical fiber 4 itself becomes unclear, so that the adjustment of high-precision amplification characteristic is impossible. Since the Er doped optical fiber 4 is kept wound around the metal reel 2, stress is always applied to the Er doped optical fiber 4, causing a reduction in reliability. While another metal reel whose diameter is adjustable is used, such a metal reel is complicated in structure and it is therefore costly.
FIGS. 2 and 3 show another EDF module 5 in the related art as applied to an EDF module which does not require temperature control. As shown in FIG. 2, an Er doped optical fiber 4 is wound around a metal reel 6. Thereafter, the Er doped optical fiber 4 is removed as a bundle from the metal reel 6. The metal reel 6 is formed with a plurality of bundling recesses 8. FIG. 3 shows the EDF module 5 removed from the metal reel 6. Reference numerals 10 denote spliced portions between the Er doped optical fiber 4 and single-mode optical fibers 12. Reference numerals 14 denote bundling tubes.
As shown in FIG. 3, the EDF module 5 is removed from the metal reel 6 for use, so that the stress always applied to the Er doped optical fiber 4 can be removed. However, it is necessary to use the metal reel 6 having such a special shape that it has the bundling recesses 8 for bundling the Er doped optical fiber 4 as shown in FIG. 2. Accordingly, there is a possibility that the coating of the Er doped optical fiber 4 may be deformed and/or damaged. Further, in adjusting the length of the Er doped optical fiber 4 or in splicing the single-mode optical fibers 12 to the opposite ends of the Er doped optical fiber 4, it is necessary to once remove the bundling tubes 14 and to bundle the Er doped optical fiber 4, so that the operation becomes troublesome. In the case that temperature control of the Er doped optical fiber 4 is necessary, the EDF module 5 shown in FIG. 3 must be stored into a container for temperature control.